Phenocryst

About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.

Experimental Evidence for Polybaric Differentiation of Primitive Arc Basalt beneath St. Vincent, Lesser Antilles

Abstract: Equilibrium crystallization experiments have been performed on a primitive high-MgO basalt (HMB) from Soufriere, St. Vincent, with three initial H2O contents (0·6, 2·3 and 4·5 wt %), at pressures of 0·4, 0·7, 1·0 and 1·3 GPa and temperatures from 1350 to 950°C. Redox conditions, as determined by µXANES analysis of Fe3+ in experimental glasses, were 1–4 log units above the nickel–nickel oxide (NNO) buffer. The aim of the study was to explore the differentiation conditions that gave rise to the observed geochemical variation in lavas and plutonic (cumulate) xenoliths from St. Vincent. An experiment with 4·5 wt % initial H2O is multiply saturated close to its liquidus (1180°C and 1·3 GPa) with a spinel lherzolite assemblage, which is consistent with a primary origin for HMB in the mantle wedge. Multiple saturation of HMB with 2·3 wt % H2O was not observed, but is inferred to occur at pressures >1·3 GPa. The experimental results show that initial H2O content has significant influence on differentiation paths of primary HMB magma, with different lava varieties generated under discrete, well-constrained P–T–H2O conditions. Low-magnesian basalts (LMB) can be generated from HMB with 2·3–4·5 wt % H2O at pressures of 1·0–1·3 GPa, corresponding to Moho depths beneath St. Vincent. The CaO contents of LMB are sensitive to differentiation pressure: high-CaO LMB are produced at pressures >0·5 GPa. Basaltic andesites (BA) can be generated at 0·7–1·0 GPa from HMB with 0·6–2·3 wt % H2O. High-alumina basalts (HAB) are produced at mid- to upper-crustal conditions (≤0·4 GPa) by differentiation of HMB with high initial H2O (≥4 wt %) through delay of plagioclase crystallization and dominant fractionation of olivine, clinopyroxene and spinel. St. Vincent andesites could be produced from relatively dry (≤0·6 wt % H2O) HMB only at lower-crustal conditions. This is suggestive of a partial melting origin from precursor HMB that had solidified at depth to produce gabbros with ∼30% hornblende (i.e. ∼0·6 wt % structurally bound H2O). The experimentally determined differentiation conditions are consistent with polybaric differentiation within a hot zone that extends from the Moho and uppermost mantle to the mid- or upper crust. Within the hot zone differentiation occurs by a combination of crystallization of HMB with 2–5 wt % H2O and partial melting of ancestral HMB gabbros. Although the experimental melts provide an excellent match to erupted lava compositions, experimental crystal compositions do not match either phenocrysts or cumulate crystals, as preserved in xenoliths. The failure to reproduce natural crystal compositions suggests that these are formed as differentiated magmas ascend and attain their H2O-saturated liquidi at shallower pressures. Thus there is a disconnect between the high-pressure phase compositions and assemblages that generate liquid compositional diversity and the low-pressure composition and assemblages that occur as phenocrysts and in cumulate xenoliths. This finding lends support to the idea of cryptic fractionation in the generation of arc magmas.

Petrologic determination of ascent rates for the 1995–1997 Soufriere Hills Volcano andesitic magma

Abstract: The recent eruption of the Soufriere Hills Volcano in Montserrat (July, 1995, to present; October, 1997) has produced a hornblende-bearing, andesitic lava dome. It is possible to petrologically estimate changes in ascent rates of amphibole-bearing magmas. For certain rates of decompression, a breakdown rim of fine-grained, anhydrous reaction products forms where amphibole is in contact with melt. The thickness of the rim varies with ascent rate. Most of the amphibole phenocrysts in the magma storage region lack breakdown rims. About 10% have 200–400 µm-thick, coarse-grained breakdown rims that are interpreted to be relicts of a past heating event. Study of a time series of new dome andesites showed that ascent rate increased from December, 1995 (∼0.001 m/s), to July, 1996 (∼0.008 m/s), while eruptive style remained extrusive. Ascent rate increased to >0.012 m/s in August, 1996, and the first major explosive eruption occurred on 17–18 September, 1996.

Etendeka Volcanism of the Goboboseb Mountains and Messum Igneous Complex, Namibia. Part II: Voluminous Quartz Latite Volcanism of the Awahab Magma System

Abstract: The Goboboseb-Messum volcanic centre is the source of two voluminous silicic eruptive sequences, the Goboboseb quartz latiites (Units I-III), and the Springbok quartz latite unit (both within the Awahab Formation). Intrusive equivalents exist as plugs and a laccolith peripheral to the Messum Complex. The recognition of correlatives of these quartz latite units in the southeastern Parana suggests eruptive volumes of ∼2320 km (Goboboseb units) and 6340 km (Springbok unit). The latter is thought to be a single eruptive event. Phenocryst assemblages are plagioclase (An), pyroxene, titanomagnetite and apatite. Pyroxene assemblages range from augite, to augite + pigeonite, to pigeonite, to pigeonite ± hypersthene, the assemblages changing progressively from the Goboboseb unit through to the Springbok unit. Although pyroxene phenocrysts from individual samples are compositionally very uniform there is a small increase in Fe through the sequence, attributed to decreasing temperature (± pressure). Thermometry suggests melt temperatures >1000°C. Many plagioclases contain abundant glass inclusions of three compositional types, thought to result from active disequilibrium melting at magma chamber walls. Relatively small, but systematic, changes in whole-rock composition occur stratigraphically from the lowest Goboboseb unit through to the Springbok unit and to their correlatives in the Parana, best shown by SiO (67-71%) and FeO* (5·4-7·4%) which increase and decrease, respectively, in the progressively younger eruptive phases. P, Ti, γ, Zr, Nb and Cu are positively correlated with FeO*, whereas e and Pb isotope compositions correlate inversely with FeO*. Crust-normalized spidergram plots indicate strong negative Sr anomalies, accompanied by significant Eu/Eu* anomalies (0·62-0·67). The quartz latite melts can be interpreted in terms of large-scale assimilation-fractional crystallization (AFC)-style processes, involving high degrees of lower- and upper-crustal melting, with thermal and material input from hybridized LTZ.L-type basaltic magmas (Part I). Thermal source is inferred to be the Tristan plume. The crustal end-member is thought to be the mid-Proterozoic restite source of the Damara granites, although some shallower crustal input is also likely. Modelling suggests the source may be similar to A-type granites and charnockites (i.e. relatively REE and HFSE enriched). Available seismic data suggest a simple velocity crustal profile, possibly the result of the massive crustal and uppermost mantle melting that accompanied the evolution of the Awahab magma system.

Tephra-fall deposits from the 1992 eruption of Crater Peak, Alaska : Implications of clast textures for eruptive processes

Abstract: The 1992 eruption of Crater Peak, Mount Spurr, Alaska, involved three subplinian tephra-producing events of similar volume and duration. The tephra consists of two dense juvenile clast types that are identified by color, one tan and one gray, of similar chemistry, mineral assemblage, and glass composition. In two of the eruptive events, the clast types are strongly stratified with tan clasts dominating the basal two thirds of the deposits and gray clasts the upper one third. Tan clasts have average densities between 1.5 and 1.7 g/cc and vesicularities (phenocryst free) of approximately 42%. Gray clasts have average densities between 2.1 and 2.3 g/cc, and vesicularities of approximately 20%; both contain abundant microlites. Average maximum plagioclase microlite lengths (13–15 μm) in gray clasts in the upper layer are similar regardless of eruptive event (and therefore the repose time between them) and are larger than average maximum plagioclase microlite lengths (9–11 μm) in the tan clasts in the lower layer. This suggests that microlite growth is a response to eruptive processes and not to magma reservoir heterogeneity or dynamics. Furthermore, we suggest that the low vesicularities of the clasts are due to syneruptive magmatic degassing resulting in microlitic growth prior to fragmentation and not to quenching of clasts by external groundwater.